In recent years, it has been desperately desired to reduce carbon dioxide emission in order to cope with air pollution and global warming. In the automobile industry, expectations have been focused on introduction of electric vehicles (EV) and hybrid electric vehicles (HEV) as means for reducing carbon dioxide emission.
As for electric vehicles, it is particularly required that the range per charge and the performance are comparable to those of gasoline vehicles. Development of secondary cells for driving a motor is a key to putting them into practice, which have been intensively conducted.
In secondary cells for driving a motor, lithium-ion secondary cells with high energy density have drawn attention and are now in rapid development. However, it has been pointed out that it is very difficult to achieve a goal with conventional technical improvements in lithium-ion secondary cells.
In this regard, metal-air cells (batteries) using zinc for the anode has drawn attention, which are considered to have a potential of achieving higher energy density than lithium-ion secondary cells.
Zinc (Zn) used in such metal-air cells are inexpensive material that is abundant on the earth and has high theoretical capacity density. It has been desired to put such secondary cells using zinc as the anode into practice because of the low cost and the potential of achieving the greatly increased energy density compared to conventional secondary cells that have been practically used.
However, a problem with zinc secondary cells using aqueous electrolytic solution is the very short charge-discharge cycle life, which is a significant hurdle that has to be overcome to put them into practice.
That is, in secondary cells using zinc as the anode active material, repetitive charge and discharge cause degradation of the cell performance such as the occurrence of an internal short circuit and a decrease of the discharge capacity due to growth of zinc dendrite, densification, shape change or the like.
By a discharge reaction of the following Chemical Equation (1), zincate anion (Zn(OH)42-) soluble to strong alkaline electrolytic solution is produced as a discharge product from zinc.Discharge Reaction: Zn+40H−→Zn(OH)42−+2e−  Chemical Equation (1)
In order to prevent such Zn components from being dissolved into the electrolytic solution, strong alkaline aqueous solution saturated with zinc oxide (ZnO) is generally used as the electrolytic solution of cells with a zinc electrode.
However, even when ZnO is saturated, zincate anion is dissolved in a supersaturated condition up to a concentration several times higher than the saturated solubility. Therefore, the zincate anion produced by discharge can be readily diffused or migrated into the electrolytic solution.
During the process of charging or discharging the anode, when the zincate anion concentration locally exceeds the supersaturated solubility, or when the supersaturated solubility of zincate anion is decreased as a result of a local decrease of the OH− concentration of the electrolytic solution, the zincate anion is deposited as solid zinc oxide by a chemical reaction of the following Chemical Equation (2).Zn(OH)42−→ZnO+H2O+2OH−  Chemical Equation (2)
That is, it is considered that the shape change of the zinc electrode proceeds since repetitive charge and discharge cause repetitive deposition and precipitation of zinc oxide as described above at the same location in the anode.
By a charge reaction in the zinc anode, zinc oxide species (ZnO or Zn(OH)42-) are electrochemically reduced to produce metal zinc.Charge Reaction 1: ZnO+H2O+2e−→Zn+2OH−  Chemical Equation (3)Charge Reaction 2: Zn(OH)42−+2e−→Zn+4OH−  Chemical Equation (4)
However, since metal zinc is soluble to the strong alkaline electrolytic solution, a hydrogen-generating dissolution reaction of the following Chemical Equation (5) occurs to cause self-discharge that consumes the produced metal zinc. As a result, the discharge capacity of the zinc anode is decreased relative to the charge capacity.Zn+2OH−+H2O→Zn(OH)42-+H2↑  Chemical Equation (5)
To suppress the self-discharge of the zinc anode caused by the hydrogen-generating dissolution reaction, measures such as usage of electrolytic solution with low alkaline concentration have been attempted. However, the decrease in ion conductivity and the increase in H2O activity of the electrolytic solution cause an increase of a hydrogen-generating side reaction during charge of the zinc electrode and the like. Since the resultant increase of the charge and discharge overvoltage leads to disadvantages of the increased energy loss and the decreased charge-discharge efficiency, it is difficult to use electrolytic solution with low alkaline concentration.
Characteristics and Electrochemical Performance of the TiO2-Coated ZnO Anode for Ni—Zn Secondary Batteries, S-H Lee et al., J. Phys. Chem. C, 115, 2572 (2011) discloses that a secondary cell using zinc oxide (ZnO) particles allegedly coated with titanium oxide (TiO2) as an active material has improved charge cycle durability than secondary cells using non-coated native zinc oxide (ZnO) particles as an active material.